Field of the Invention
This invention relates to a method and apparatus for purifying oxygen-containing gas streams. More particularly the invention pertains to a process for producing substantially oxygen-free inert gas streams, such as oxygen-free argon, nitrogen, helium and neon.
Inert gases, such as argon, nitrogen, and the like are widely used in industrial processes such as shielded arc welding, semiconductor manufacture, metal refining, electric light bulb manufacture and inert gas-blanketed chemical processes. In many industrial processes the presence of oxygen as an impurity in the inert gas frequently brings about undesirable results, such as the formation of oxides of materials treated in the process. Accordingly, it is frequently necessary that the inert gases used in these processes be substantially free of oxygen.
Crude inert gases that are separated from air by cryogenic or non-cryogenic means, such as argon and nitrogen, ordinarily contain up to several percent by volume of oxygen. For example, argon manufactured from air by cryogenic distillation usually contains up to 3% oxygen. This happens because it is very difficult to completely separate argon and oxygen by cryogenic distillation, since argon and oxygen have very close boiling points. When it is desired to produce substantially oxygen-free argon from oxygen-containing sources such as air, it is generally necessary to resort to purification processes other than cryogenic distillation. Methods for manufacturing high purity oxygen-free argon and other high purity inert gases include adsorption by means of molecular sieves, and catalytic deoxygenation (oxygen-removal) processes, such as the catalytic oxidation of oxygen- and hydrogen-containing streams and chemisorption of the oxygen by getter materials. Catalytic processes are generally preferred over adsorption because they provide superior results and have low operating costs.
A preferred catalytic process for the removal of oxygen from gas streams involves adding hydrogen to the gas stream and subsequently contacting the gas stream with an oxidation catalyst, for example, a noble metal catalyst, such as platinum or palladium. The catalyst converts the oxygen and hydrogen into water, which can be subsequently removed. Such a process is disclosed in U.S. Pat. No. 4,960,579, which teaches the production of high purity nitrogen from air by first separating nitrogen from air by membrane separation or pressure swing adsorption and then removing residual oxygen from the nitrogen product by introducing purified hydrogen into the gas stream and contacting the stream with an oxidation catalyst such as a noble metal or combination of noble metals to cause the oxygen and hydrogen in the stream to combine to form water. Other patents which disclose contacting oxygen- and hydrogen-containing gas streams with a deoxygenation catalyst are U.S. Pat. Nos. 3,535,074, 4,579,723 and 4,713,224.
This above-described catalytic deoxygenation process performs satisfactorily with fresh catalyst; however the concentration of oxygen impurity in the product gas can increase over extended periods of time. It appears that the catalyst gradually deteriorates upon continued usage. It has been theorized that the deterioration of the catalyst is caused by the presence of catalyst poisons that are present in the gas stream being treated. These impurities may come from various sources. One likely source of impurities is the gas stream entering the system for purification. This stream may contain trace amounts of gaseous impurities, such as sulfur compounds. Another possible source of impurities is the water that is used to cool the feed gas compressor commonly used in conjunction with the process. The cooling water may contain elements or compounds, such as chlorine or compounds of chlorine, phosphorus and molybdenum that were initially present in the water or that were added in water treatment operations. The above impurities are known catalyst poisons. Even though they are present in the gas stream in very small concentrations, long term exposure of the deoxygenation catalyst to them will slowly cause the catalyst to become poisoned. In addition to catalyst poisons, moisture present in the gas stream may cause a reduction of the oxidation activity of the catalyst.
Process improvements which eliminate or reduce the adverse impact of poisons on the effectiveness of deoxygenation catalysts used in the production of oxygen-free inert gases are constantly sought. The present invention provides such an improvement.